The invention relates to the production and maintenance of predetermined proportionate mixtures of fluids, and more specifically to a system and method for producing and maintaining a proportionately constant mixture of water and foam concentrate, typically for use in firefighting apparatus.
Direct injection foam concentrate proportioning systems offer many advantages over the currently used balanced pressure, in-line eductor and around-the-pump systems. Balanced pressure systems require ratio controllers which have limits as to flow range, and which also typically produce an undesirable pressure loss. In-line eductor systems are limited as to flow range and back pressure tolerance, and typically cause a pressure loss of 30-40% of inlet pressure. Due to the fact that eductors will not function if back pressure exceeds 65-70% of inlet pressure, these systems must be carefully matched to the downstream system so that excessive back pressure will not occur. Their proportioning accuracy is poor if the inlet pressure varies significantly from the design value. Around-the-pump systems require readjustments of the proportioning valve each time the system flow changes, and will not work if pump inlet pressure exceeds the allowable back pressure of the high volume jet pump used in the system. The around-the-pump proportioners cannot be used in systems where foam solution is desired at some system discharge points and water at others. However, they have considerable utility, especially in aircraft crash rescue vehicles and municipal pumpers.
Conventional direct injection systems make use of paddlewheel-type flow meters to measure water and foam concentrate flow rates. The foam concentrate flow rate is then adjusted either manually or automatically to the desired percentage of the water flow rate. Paddlewheel flow meters are affected by the viscosity of the liquid being measured, thus making it necessary to recalibrate them for foam concentrates of differing viscosities, and making them impossible to use with foam concentrates which have non-Newtonian rheologies. Exemplary systems of this type are described in U.S. Pat. Nos. 4,324,294 and 4,246,969, both issued to McLoughlin et al., relating to chemical injection systems.